The present invention relates to electronic amplifier systems and, in particular, to such a system which is exceptionally well adapted for use in instruments utilized in medical diagnoses and monitoring of physiological functions. This novel amplifier system exhibits the normal condition stability of AC coupling, yet is capable of providing the effective recovery from a broad range of overload conditions which has heretofore been available only with the less desirable DC coupled amplifiers.
Because of their ability to reject undesirable input signal DC components arising, for example, from electrode offset potentials or long term DC drifts, AC coupled amplifiers are employed for virtually all routine diagnostic and monitoring applications involving the measurement of electrical potentials associated with the human heart. However, in order to faithfully reproduce these electrical signals, the time constants associated with the AC coupling capacitors in the signal processing amplifier must be relatively long. For example, time constants which yield a low frequency 3 dB point between about 0.5 Hz and 0.05 Hz are commonly used in ECG monitoring and routine diagnostic ECG equipments.
Unfortunately, the use of such long time constants can result in undesirable amplifier response should the amplifier be driven into overload. Such amplifier overload response may vary from disturbances of the normal signal which severely limit its clinical usefulness to complete loss of the normal signal for several seconds. In addition, these undesired responses may cause malfunctions in additional processing circuitry such as heart rate meters, alarm sensing circuits, signal display scopes, and hard copy recorders.
In actual practice, ECG amplifiers are quite frequently driven into overload conditions. Signals which produce amplifier overload can be broadly classified into two groups; the short, transient type such as may arise from a pacemaker pulse or defibrillator discharge, or the longer term, extended type which may result from electrode recovery following a defibrillator discharge or the presence of a sustained overscale electrode offset potential. Whichever the type, it is evident that the disturbance of the charge on the AC coupling capacitor of the amplifier from its nominal value by the overload signal is the primary factor which results in the undesired residual amplifier response after the overload signal passes.
Previously available amplifier systems have been generally designed to deal with the two broad classes of overload signals by means of distinctly different circuitry, each optimized to handle separately one or the other class of overload signal. More specifically, signal overloads of a short or transient nature have usually been processed through slew rate limiter circuits which limit the amount of charge disturbance on the coupling capacitor by controlling the maximum rate of change at which charge can be either increased or decreased in the capacitor. Signal overloads of a longer duration have normally been dealt with by various circuit means which either modify coupling time constants or provide controlled charge establishment on the coupling capacitor after a specific interval of time.
While slew rate limiting is an effective means for suppressing transient disturbances, it has various inherent disadvantages which restrict its utility. For example, slew rate limiting forces a compromise between the high frequency signal handling ability of the amplifier and the amount of transient suppression desired. This represents a definite disadvantage, since reproduction of the higher frequency components is desirable for certain clinically encountered heart potentials such as large amplitude, rapidly changing signals associated with pediatric patients, neonatal patients, and certain invasive measurements on adult patients.
Further, since the rate of change in decreasing signal level is no less affected, slew rate limiting produces a stretching effect for pulse type overloads which essentially doubles the width of pacemaker "spikes" and aggravates the problem of distinguishing in heart rate meter circuits between such a "spike" and certain narrow QRS complexes in adults.
In addition, if slew rate limiting is applied to the degree necessary to suppress to a negligible level any baseline disturbance of the normal ECG signal, a pacemaker "spike" is so suppressed that it is difficult to determine the temporal relationship between the "spike" and the ECG signal. The ability to "see" the pacemaker "spike" without undue disturbance of the normal ECG signal or the heart rate counting circuitry is particularly important for diagnostic procedures and research studies such as pacemaker-cardiac capture mechanisms or certain high rate atrial pacing techniques.
On the other hand, charge re-establishment circuits are severely limited in utility to the specific overload conditions for which they are designed. For example, if the duration of the overload is longer than a brief transient but shorter than the specific time interval in which such a circuit acts to compensate for a charge disturbance, the charge on the coupling capacitor may be disturbed to such an extent that the normal signal is displaced outside the operating range of associated processing apparatus such as display scopes, hard copy recorders, or heart rate meters. When such a condition occurs it can take several seconds for the signal to return to a range where these peripheral devices can provide useful data. Also, such circuits seldom provide well-defined amplifier output response during the period of time the overload persists. As a result of such a lack of defined output level subsequent signal processing circuits can be overloaded. Further lacking are means for controlling charge compensation in proportion to the gain or scale factor chosen by the equipment operator. Such fixed compensation circuitry thus results in much longer recovery times whenever the gain or scale factor of the amplifier is increased.